Various machine readable symbols have been developed for many applications. For example, the Universal Product Code (UPC) is a bar code symbology widely used in the U.S. retail industry to identify products at the point of sale, or for inventory control purposes. In the industrial area, other bar code symbologies have been used for package identification systems. Common bar code symbologies include CODABAR, code 39,interleaved 2 of 5, and code 49. In general, machine readable codes provide significant advantages in the efficiency of material handling and data gathering.
A bar code is a particular type of machine readable symbol. A typical bar code includes parallel solid lines, or bars, of varying width and spacing. The alternating light and dark areas defined by the bars and the spaces between the bars, represent a digital code which serves to identify the content of the bar code symbol. After being read, the digital code is then directly translated to a sequence of alphanumeric characters and then by means of a data base, may be further translated to the common language description of the item bearing the subject bar code label, along with other pertinent data, such as for example the current price of the item.
A bar code may be read by scanning. Typically, a small spot of light is directed from a source of illumination to the surface of the bar code. The reflected light is sensed by a photosensitive element. The small illuminated spot of light is then swept across the bar code surface, all the while sensing the intensity of the resulting reflected light. Since light areas reflect more light than dark areas, the reflected light intensity represents the digital code which serves to identify the content of the bar code symbol.
In one type of bar code reader, a hand held laser or LED is used as the source of illumination, and the reader is manually swept across the bar code surface. In a scanning type of bar code reader, the light source is stationary while the light beam is moved in a scanning pattern. A typical scan pattern may be a linear bidirectional sweep. In the latter type of scanner, the bar code symbol and the bar code reader must still be manually oriented so that the scan pattern traverses all the bars of the bar code in order for the bar code to be properly scanned and read.
In another type of bar code scanner, a laser beam is swept through a complex series of repetitive geometric patterns in order to provide for some degree of omnidirectional scanning. Omnidirectional laser based scanners tend to be expensive due to the initial cost and limited lifetime of the laser light source. Also, a laser having an intense concentrated light source can be an eye hazard, and requires special precautions in manufacturing and handling.
Also important, a laser scanner requires a complex mechanism to sweep the laser beam in a predetermined pattern. The resulting scanning mechanism includes complex optical elements such as rapidly rotating polygonal mirrors. Components must be able to withstand both mechanical wear, as well as maintain optical properties under difficult conditions, as may be encountered in industrial environments. Thus, mechanical laser scanners tend to be expensive due to the cost of precision optical and mechanical scanning mechanisms.
Furthermore, as faster scanning speeds are required, the problems associated with mechanical scanners become more acute. For example, at very high scanning speeds, mechanism wear may be excessive. More important, as scanning speed is increased, laser power must be increased in order to maintain the same illumination energy density. Also, complex scanning patterns add to the complexity of the scanning mechanism. In many situations involving high scanning speeds and/or complex scanning patterns, mechanical scanning may be impractical or impossible.